jason ferguson is ideal for the 3-4. and im surprised no one said this: shawn merriman is perfect for your 3-4 defense! unless you cant afford him, keep him.
3-4 uses basically 3 tackles as linemen. 285lbs and up, preferably at leas 300lbs. your outside linebackers should be around 250-270 lbs. look for a good combo of size, speed and agility. these will be your best athletes. a very athletic 4-3 de can be converted to olb. you want to look at the block shedding rating as well for your backers, trust me. your two ilb's are mainly run stuffers, and i would try to have at least one of them be very athletic. looking forward to seeing how this works out for you